Electrostatic deflection arrangement for electron tubes



July 28, 1964 G. WENDT 3,142,779

ELECTROSTATIC DEFLECTION ARRANGEMENT FOR auzcmou mass 2 Sheets-Sheet 1 Filed Dec. 14, 1959 I I I 2 f I V 22 2| 8 14 swzzv PARABOLIC csnzmwon VOLTAGC I 7 GCNERATOR 1e 14 Fig.1

INVENTOR qzonc WENDY BY 7 ATTORNEY? e. WENDT 3,142,779

ELECTROSTATIC DEFLECTION ARRANGEMENT FOR ELECTRON TUBES Filed Dec. 14, 1959 2 Sheets-Sheet 2 INVENTOR GEORG WEHDT 3,142,77 ELECTRQSTATHC DEFLE'CTIGN ARRANGEMENT FOR ELEQTRGN TUBES Georg Wendt, Paris, France, assignor to Compagnie Generaie de Telegraphic Sans Fit, Paris, France Filed Dec. 14, 1959, Ser. No. 859,267 Claims priority, application France Ben. 12, 1957 3 Ciaiins. (Cl. 315-22) The present application is a continuation-in-part application of my copending application Ser. No. 777,382, filed December 1, 1958, now abandoned, and entitled Electrostatic Deflection Arrangement for Electron Tubes.

The present invention relates to electron tubes provided with electrostatic deflection for an electron beam, and more particular to improvements providing a compensation for tubes with electrostatic deflection for the electron beam propagating within the tube.

It is known in the prior art that in tubes provided with electrostatic deflection of a beam, the resolution of the image inscribed by the spot of the beam on a screen is defective by reason of the astigmatism of the electronoptical system which includes two pairs of deflection plates disposed essentially at right angle to each other. This astigmatism becomes noticeable by the fact that the cross section of the beam, and therewith the form or shape of the spot varies according to the position of the latter on the screen, and, instead of remaining circular, becomes flattened in one or the other direction dependent on the position thereof on the screen, that is according to the voltage applied to the deflection plates.

This defect might possibly be corrected in the first instance by acting on the geometry of the system of the deflection plates. But this solution, if applicable to the first set of plates, i.e., to the set adjacent the electron gun, where the beam sweeps within a single plane, is very difficult in the application thereof to the second set of plates, i.e., to the set adjacent the screen where the beam is displaced in two mutually perpendicular dimensions, that is, sweeps a volume in space, and where each change or modification in the form of the plates or the distance thereof exercises on the beam certain efiects of which the reciprocal interdependences thereof are very complex. Consequently, by attempting to correct, in this manner, the aberration due to astigmatism of the second system or set of plates, other inadmissible deformations are inevitably and unavoidably introduced into the spot.

One of the primary objects of the present invention is a tube with electrostatic deflection in which the astigmatism is corrected by simple means which do not, in themselves, constitute a new source of aberration.

In the tube according to the present invention, a diaphragrn provided with an aperture or opening of geometrically simple form, for example, provided with a circular hole or bore, is placed into the electron gun of the tube, between the input orifice of the tube within the casing which encloses the systems of deflection plates and a rectilinear gap, slit, or slot disposed in front of the input of the first system or set of plates or constituting this input, which diaphragm together with this gap, slit or slot constitutes an electrostatic lens of which the field of the electric lines of force presents two orthogonal planes of symmetry and which has difierent convergences along the two dimensions thereof, the direction of minimum convergence corresponding to the direction of deflection given to the beam by the second pair of plates. This diaphragm is carried at a voltage proportional, at every instant, to the square of the voltage applied to the second pair of plates. If this voltage is variable in time, for example, if this voltage varies in time proportionally within the limits of a given period of sweep, the voltage BJiZJYQ Patented July 28, 1964 or potential of the diaphragm will, therefore, assume a parabolic shape between the beginning and the end of this period, the variation thereof repeating itself periodically at the same time as the sweep. Additionally, a second diaphragm in the form of an apertured metallic partition or shield is provided between the two sets of deflection plates. The geometric configuration of the aperture in the second diaphragm which is essentially rectangular and essentially equal to or somewhat greater than the rectangle formed by the edges at the input of the second set of deflection plates is of great importance in achieving the desired aims by simple means.

It is an object of the present invention to provide a correction system for the electrostatic deflection system of an electron beam which corrects effectively the aberrations thereof due to astigmatism.

t is another object of the present invention to provide a correction system for an electron tube having a pair of electrostatic deflection plates which is simple in construction, reliable in performance, easy to install, and which does not increase the length of the tube.

These and other objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, one embodiment in accordance with the present invention and wherein:

FIGURE 1 is a somewhat schematic longitudinal crosssectional view through the electron gun of an electron tube provided with a correction system in accordance with the present invention,

FIGURE 2 is a partial perspective view of FIGURE 1, and

FIGURE 3 is a diagram indicating the voltage applied to the correction system in accordance with the present invention and plotted against time.

Referring now to the drawing and more particularly to FIGURES 1 and 2, the different electrodes and elements of an electron gun of classical construction for a tube provided with electrostatic deflection are shown therein which includes a cathode surrounded by a Wehnelt electrode 1, a first anode composed of two cylinders 2 and 3, a collimation lens 4, and a second anode constituted by a cylinder 6 connected to the casing 7 which surrounds a first system of deflection plates 8 and a second system of deflection plates 9. The two sets of plates 8 and 9 are supported within the casing 7 by insulating discs 5. An orifice or aperture it for the passage of the beam is provided in the bottom of the casing 7 and is arranged essentially coaxially with the system. A diaphragm 19 connected to the walls of the casing '7 and provided with a slit, slot or gap ill coincidental with the input of the plate system 8 is disposed in front thereof. The second anode 6 is preferably connected to the metallization of the interior walls of the tube (not shown herein) and is thus carried at the potential of the latter.

A diaphragm 12 supported by an insulating disc 18 (FIGURE 1) provided, for example, with an axial circular hole or bore 13 (FIGURES 1 and 2) is arranged within the casing 7 between the orifice l0 and the rectilinear slot 11 and is located in front of the input to the plate system 8. The circular axial hole 13 is of a diameter slightly larger or at least equal to that of the orifice it? if the beam completely fills the latter. This aperture or hole 13 and the gap 11 determine an electrostatic lens of the beam. The field of the lines of force of this lens present two orthogonal planes of symmetry oriented along the planes of symmetry of the two plate systems 8 and 9, and the convergence of this lens varies in different azimuths, being minimum in the plane of deflection of the beam under the action of the plates 9 and maximum in the plane perpendicular thereto. The diaphragm 12 is carried at a potential or voltage proportional at every instant to the square of the voltage supplied to the plates 9 by means of the generator 14 and the connection 15. If the voltage applied to the plates 9 is a sweep voltage of saw-tooth shape, the generator 14- delivers a voltage of parabolic form shown in full line in FIGURE 3. This generator 14 and therewith the parabolic voltage produced thereby is synchronized by any suitable, well-known means from the generator 16 which delivers the saw-tooth voltage, symmetrical about a point that is placed at the potential of the casing 7 for the sweep voltage of the second system of plates 9.

FIGURE 3 is a diagram of voltage as a function of time, which shows also the position of the voltage curve of the generator 14 with respect to the voltage of the generator 16, the latter being indicated in FIGURE 3 in dash line. If the metallization of the tube is taken as zero reference voltage or zero reference potential, represented by the axis of abscissa of the diagram of FIG- URE 3, the voltage of saw-tooth shape of the generator 16 is symmetrical with respect to this reference level. The mean potential of the two plates 9 is therefore zero. In contradistinction thereto, the potential of the diaphragm 12 is zero only at the moment at which the voltage of saw-tooth shape intersects the axis of abscissa, that is, when the voltages of the two plates 9 are equal and the beam remains within the mean or average plane therebetween. For all the other values of saw-tooth voltage, the potential of the diaphragm 12 is no longer zero but deviates from the axis of abscissa in a single direction along the parabolic curve shown in full line.

It should be noted that the application of such a variable potential is already known insofar as a collimation lens is concerned of the type of lens 4 of FIGURES 1 and 2, for the purpose of correcting the aberration due to the curvature of the field. Therefore, the diaphragm 12 carried at this variable potential or voltage, in accordance with the present invent-ion, may use the same voltage supply as this collimating lens to which is supplied, as known, a potential of analogous shape. This supply has been indicated in FIGURES 1 and 2 by the connection 17 connecting the generator 14 with the lens 4.

The curvature of the parabolic Voltage supplied by the generator 14, that is the exponent of the function representing the parabola is fixed by the nature of the generator and, in principle, is not regulatable or adjustable. In contradistinction thereto, the maximum amplitude of the parabolic voltage at the ends of the period, indicated in FIGURE 3 by AB, that is the coefficient of the function is adjustable or regulatable and may receive, as needed, different appropriate voltages adapted to be supplied respectively to the electrodes 4 and 12. This adjustment which may be of any well known type has been indicated symbolically in the drawings by the arrows at the ends of the connections 15 and 17 and may be realized in any conventional manner. However, if the conditions are such that these values are very little diflerent for a satisfactory overall result, it may be sufficient to use an approximation and a single voltage could be used for feeding both the lens 4 and the diaphragm 12.

In order to achieve fully and satisfactorily the objects of the present invention, i.e., the correction of the astigmatism by the single correcting means, 14, 15, 12, 13, a second apertured metallic partition or shield electrode 20 constituting a diaphragm is provided between the two sets of deflecting plates 8 and 9. The diaphragm 20 is connected to the walls of easing 7 and is provided with a rectangular aperture 21 the sides of which are equal to or somewhat greater than those of the rectangle formed by the edges of plates 9 at the input side thereof.

It has been determined experimentally that the shape of aperture 21 in diaphragm 20 is of great importance for purposes of achieving the correction of the astigmatism by simple means. This so because, for example,

the provision of an electrode 22 with a circular aperture therein would require two separate correcting means for satisfactorily correcting the astigmatism due to both the first and second set of deflecting plates 8 and 9 respectively, whereas the provision of a rectangular aperture 21 in electrode 2%}, as disclosed herein, obviates the necessity for a double correcting means, and instead makes possible a practically complete elimination of the astigmatism by providing a correction only for the astigmatism of the second set of deflection plates 9 by means of parabolic voltage generator 14 applying a corrective signal to diaphragm 12.

It has been discovered empirically in the course of the present invention that the necessity for a double correcting means of the astigmatism due to the two sets of deflection plates, as illustrated, for example, in FIGURE 7 of U. S. Patent 2,572,861 to R. G. E. Hutter may be dispensed with and that the arrangement and circuitry may thereby be considerably simplified by the use of the teachings in accordance with the present invention in which a practically complete elimination of the astigmatism due to both sets of deflection plates 8 and 9 is realized by the use of a rectangular aperture 21 in diaphragm 29 even though only corrective signals for correcting the astigmatism of the second set of deflection plates 9 are utilized thereby obviating the need of corrective signals for correcting the astigmatism of the first set of deflection plates.

I claim:

1. A cathode ray tube arrangement comprising means constituting a source of an electron beam, two pairs of mutually crossed plates positioned along the path of said beam for e'lect-rostatically deflecting the same, a potential source connected to one of said pair of plates remote from said source to apply a deflecting potential thereto, electrostatic lens means having a field of electric lines of force with two orthogonal planes of symmetry and a different convergence along said two planes, said lens means being operatively positioned along the path of said beam between said source and the other of said pair of plates adjacent said source, means for applying to said lens means a potential proportional at each instant to the square of said deflecting potential applied to said one pair of plates to correct for the astigmatism caused by said one pair, diaphragm means in the form of an electrode intermediate said two pairs of mutually crossed plates and provided with a rectangular aperture having two sides thereof at least equal to the edges of said one pair of plates at the input thereof to thereby correct the astigmatism of the other pair of plates, said electrostatic lens means being defined, on the one hand, by a slit between the two plates of said other pair of plates, and, on the other hand, by an aperture for the passage of said beam provided in a diaphragm disposed between said source and said other pair of plates, said means for applying said proportional potential being operatively connected to said diaphragm, and wherein said last-mentioned aperture is of circular shape. 7

2. A cathode ray tube arrangement comprising means constituting a source of an electron beam, two pairs of mutually crossed plates positioned along the path of said beam for electrostatically deflecting the same, a potential source connected to one of said pair of plates remote from said source to apply a deflecting potential thereto, electrostatic lens means having a field of electric lines of force with two orthogonal planes of symmetry and a different convergence along said two planes, said lens means being operatively positioned along the path of said beam between said source and the other of said pair of plates adjacent said source, means for applying to said lens means a potential proportional at each instant to the square of said deflecting potential applied to said one pair of plates to correct for the astigmatism caused by said one pair, diaphragm means in the form of an electrode intermediate said two pairs of mutually crossed plates and provided with a rectangular aperture having two sides thereof at least equal to the edges of said one pair of plates at the input thereof to thereby correct for the astigmatism of the other pair of plates, said electrostatic lens means being defined, on the one hand, by a slit between the two plates of said other pair of plates, and, on the other hand, by an aperture for the passage of said means provided in a diaphragm disposed between said source and said other pair of plates, said means for applying said proportional potential being operatively connected to said diaphragm, a second diaphragm disposed between said aperture diaphragm and said other pair of plates, a slit being provided in said second diaphragm which is essentially parallel to the slit of said other pair, and means for applying a fixed potential to said second diaphragm.

3. A cathode ray tube arrangement comprising means constituting a source of an electron beam, two pairs of mutually crossed plates positioned along the path of said beam for electrostatically deflecting the same, a potential source connected to one of said pair of plates remote from said source to apply a deflecting potential thereto, electrostatic lens means having a field of electric lines of force with two orthogonal planes of symmetry and a difierent convergence along said two planes, said lens means being operatively positioned along the path of said beam between said source and the other of said pair of plates adjacent said source, means for applying to said lens means a potential proportional at each instant to the square of said deflecting potential applied to said one pair of plates to correct for the astigmatism caused by said one pair, diaphragm means in the form of an electrode intermediate said two pairs of mutually crossed plates and provided with a rectangular aperture having two sides thereof at least equal to the edges of said one pair of plates at the input thereof to thereby correct for the astigmatism of the other pair of plates, a collimating lens disposed between said source and said electrostatic lens means, and means for applying to said collimating lens a potential proportional at each instant to the potential applied to said electrostatic lens means.

References Cited in the file of this patent UNITED STATES PATENTS 2,185,239 Von Ardenne Jan. 2, 1940 2,572,858 Harrison Oct. 30, 1951 2,572,861 Hutter Oct. 30, 1951 2,884,559 Cooper Apr. 28, 1959 2,951,961 Cooper Sept. 6, 1960 

1. A CATHODE RAY TUBE ARRANGEMENT COMPRISING MEANS CONSTITUTING A SOURCE OF AN ELECTRON BEAM, TWO PAIRS OF MUTUALLY CROSSED PLATES POSITIONED ALONG THE PATH OF SAID BEAM FOR ELECTROSTATICALLY DEFLECTING THE SAME, A POTENTIAL SOURCE CONNECTED TO ONE OF SAID PAIR OF PLATES REMOTE FROM SAID SOURCE TO APPLY A DEFLECTING POTENTIAL THERETO, ELECTROSTATIC LENS MEANS HAVING A FIELD OF ELECTRIC LINES OF FORCE WITH TWO ORTHOGONAL PLANES OF SYMMETRY AND A DIFFERENT CONVERGENCE ALONG SAID TWO PLANES, SAID LENS MEANS BEING OPERATIVELY POSITIONED ALONG THE PATH OF SAID BEAM BETWEEN SAID SOURCE AND THE OTHER OF SAID PAIR OF PLATES ADJACENT SAID SOURCE, MEANS FOR APPLYING TO SAID LENS MEANS A POTENTIAL PROPORTIONAL AT EACH INSTANT TO THE SQUARE OF SAID DEFLECTING POTENTIAL APPLIED TO SAID ONE PAIR OF PLATES TO CORRECT FOR THE ASTIGMATISM CAUSED BY SAID ONE PAIR, DIAPHRAGM MEANS IN THE FORM OF AN ELECTRODE INTERMEDIATE SAID TWO PAIRS OF MUTUALLY CROSSED PLATES AND PROVIDED WITH A RECTANGULAR APERTURE HAVING TWO SIDES THEREOF AT LEAST EQUAL TO THE EDGES OF SAID ONE PAIR OF PLATES AT THE INPUT THEREOF TO THEREBY CORRECT THE ASTIGMATISM OF THE OTHER PAIR OF PLATES, SAID ELECTROSTATIC LENS MEANS BEING DEFINED, ON THE ONE HAND, BY A SLIT BETWEEN THE TWO PLATES OF SAID OTHER PAIR OF PLATES, AND, ON THE OTHER HAND, BY AN APERTURE FOR THE PASSAGE OF SAID BEAM PROVIDED IN A DIAPHRAGM DISPOSED BETWEEN SAID SOURCE AND SAID OTHER PAIR OF PLATES, SAID MEANS FOR APPLYING SAID PROPORTIONAL POTENTIAL BEING OPERATIVELY CONNECTED TO SAID DIAPHRAGM, AND WHEREIN SAID LAST-MENTIONED APERTURE IS OF CIRCULAR SHAPE. 